Magnetic flow meter

ABSTRACT

A magnetic flow meter, the pre-amplifier of which comprises two identically connected operational amplifiers having differential inputs, each feeding one of the differential inputs of a third operational amplifier, two identical impedances connected in series between the inverting inputs of the first two operational amplifiers, a fourth operational amplifier connected as a voltage follower the input of which is connected to the node of the said two impedances and the output of which is connected through two pairs of series resistors to the corresponding non-inverting inputs of the two first mentioned operational amplifiers, the nodes of said pairs of series resistors being connected through further resistors to the output of the corresponding operational amplifiers, the input terminals of the pre-amplifier being formed by the said non-inverting inputs of the two first mentioned operational amplifiers and the output of the pre-amplifier being formed by the output of the third operational amplifier.

[ Mar. 27, 1973 MAGNETIC FLOW METER [75] Inventors: Luc Yves NatensBerchem; Jean Martha De Gueldre, Edegem, both of Belgium [73] Assignee:Agfa-Gevaert N.V., Mortsel, Belgi- 22 Filed: Apr. 13, 1971 21 AppllNo;133,560

[30] Foreign Application Priority Data Primary Examiner-Charles A. RuehlAttorney-Willia.m J. Daniel [57] ABSTRACT A magnetic flow meter; thepre-amplifier of which comprises two identically connected operationalamplifiers having differential inputs, each feeding one of thedifierential inputs of a third operational amplifier, two identicalimpedances connected in series between the inverting inputs of the firsttwo operational amplifiers, a fourth operational amplifier connected asa voltage follower the input of which is connected to the node of thesaid two impedances and the output of which is connected through twopairs of series resistors to the corresponding non-inverting inputs ofthe two first mentioned operational amplifiers, the nodes of said pairsof series resistors being connected through further resistors to theoutput of the corresponding operational amplifiers, the input terminalsof the pre-amplifier being formed by the said non-inverting inputs ofthe two first mentioned operational amplifiers and the output of thepre-amplifier being formed by the output of the third operationalamplifi- 5 Claims, 5 Drawing Figures MAGNETIC FLOW METER The inventionrelates to a magnetic flow meter for measuring the flow rate of anelectrically conductive liquid passing through a conduit, comprisingmeans for generating an alternating magnetic flux normal to the flowdirection of the liquid. I

The principle of the magnetic flow meter is based on Lenzs law teachingthat over a conductor moving in a magnetic field an E.M.F. is createdwhich is directly proportional to the magnitude of the field, to thevelocity of the movement and to the length of the conductor. When allparameters besides the velocity are kept to a constant value, then anunequivocal relation is obtained between the velocity and the generatedvoltage. The same reasoning also counts when the moving conductor isreplaced by an electrolite which is in galvanic contact with a set ofelectrodes.

The relation is U S.B.L.v,, wherein U the generated voltage S thesensitivity of the flow meter B magnetic field,

L length of the conductor,

11, the average flow velocity of the liquid. When the distribution ofvelocity is constant over the entire cross section of the liquid column,then S 1.

From practice it is known that the sensitivity S remains constant whenthe distribution of velocity in the conduit shows axial symmetry, whichcondition can nearly always be realized.

For avoiding disturbance by polarization phenomenons between themeasuring electrodes, the magnetic field used is an A.C. field.

When U is measured and if no error voltages are present, i.e. voltageswhich are not directly proportional to the rate of flow, then v, can bedetermined.

It is known to use a flow meter with a so-called transverse field. Sucha flow meter comprises a straight tube made from an insulating materialwith circular cross,

section. The measuring electrodes are formed by two platinum wires whichextend through opposite sides of the wall of the tube with theirextremities lying flush with the innerside of the tube and having asurface of about 1 sq.mm. The magnetic A.C. field can be generated by acoil, arranged round a magnetic core, the tube being located in an airgap of the said core.

The electrodes, the connection wires of the electrodes, and theelectrolite between the electrodes form a closed loop, and thus avoltage is generated in this loop by the A.C. magnetic field. Thisvoltage is given by lnd b and since B B sin 0) t ind= *dmw COS wt= wSin(wtg) So, this voltage U is shifted in phase in respect to thevoltage U over the measuring electrodes, which voltage is directlyproportional to B, so that these voltages can be separated very easilyfrom each other by synchronous demodulation. In an other way it ispossible to reject the induced voltage U by arranging the circuit of theliquid and the measuring leads so that: v

.9 ind The voltage over the measuring electrodes is amplified in apre-amplifier. Since the flowing liquid to be measured may have animpedance between the two electrodes which can be represented by e.g. aresistor of about 500 kfl, one may use a pre-amplifier having a veryhigh input impedance and which is arranged as close as possible to themeasuring electrodes.

Further desirable properties for the pre-amplifier include a goodsuppression of a signal in common mode and, moreover, an insensitivityfor ambient disturbances, i.e. temperature, humidity, magnetic andelectric fields.

Further, the bias current of the input stage of the amplifier must betaken in account so that a low impedance path for DC. must be providedwhile for A.C. a high input impedance is needed, as well for the voltagein differential as in common mode operation.

By the term common mode voltage as used in the present application ismeant the voltage ofthe liquid at the location of the measuringelectrodes with respect to a reference voltage such as e.g. groundvoltage. The conduit line through which the liquid to be measured passesis usually grounded at some point but the voltage at the location of themeasuring electrodes may differ from ground voltage because of undesiredmagnetic, capacitive or galvanic coupling.

By the term differential mode is meant the voltage difference betweenthe two measuring electrodes, which difference is actually proportionalto the flow rate of the liquid.

The invention relates to a magnetic flow. meter for measuring the rateof flow of a liquid flowing through a conduit, wherein a magneticalternatingv flux is generated almost normal to the direction of flow ofthe liquid through the conduit, and a voltage is measured over twomeasurement electrodes which are located on an axis normal to thedirection of the flux and to the direction of the flow liquid. The metercomprises an electronic circuit including a preamplifier having a highcommon mode rejection ratio and high input impedance, said preamplifiercomprising two identically connected operational amplifiers havingdifferential inputs, each feeding one of the differential inputs of athird operational amplifier, two identical impedances connected inseries between the inverting inputs of the first two operationalamplifiers, a fourth operational amplifier connected as a voltagefollower with its input connected to the node of the two impedances andits output connected through two pairs of series resistors to thecorresponding non-inverting inputs of the two first mentionedoperational amplifiers, the nodes of said pairs of series resistorsbeing connected through further resistors to the output of thecorresponding operational amplifiers, the input terminals of thepreamplifier being formed by the non-inverting inputs of the two firstmentioned operational amplifiers and the output of the pre-amplifierbeing formed by the output of the third operational amplifier.

The invention will now be described hereinafter by way of example withreference to the accompanying drawings.

FIG. 1 shows diagrammatically a cross-section of a flow meter with atransverse magnetic field.

FIG. 2 shows a simplified illustration of the pre-amplifier according tothe invention.

FIG. 3 is an embodiment of the pre-amplifier according to the invention.

FIG. 4 is a further embodiment of the invention.

FIG. 5 shows a block diagram of a device for measuring the difference inrate of flow with respect to a set value.

The magnetic flow meter represented in FIG. 1 is designed for measuringthe flow rates of e.g. 100 cos/min to 2 l/min and has an inside diameterof 6 mm so that a minimum flow speed of about 1 m/min is obtained. Thisspeed represents a good compromise between, at one hand, undesirable lowspeeds wherein the parasitic effects are mainly of electronic nature,and, at the other hand, the high speeds wherein asymmetric flow andturbulence cause errors.

The magnetic flow meter 85 which is used together with the pre-amplifieraccording to the invention comprises a magnetic core 84 made of thinferromagnetic laminations, insulated one from another and cementedtogether, so that the magnetic losses are kept as small as possible. Theliquid to be measured flows through a tube 86 made from an insulatingmaterial which is dimensionally stable and corrosion resistant, such ase.g. Teflon (registered trade mark) or glass, and re-enforced at theinlet and the outlet by meansof a stainless steel tube. To obtain asmall pressure drop, the tube ends are conically finished.

In the tube 86 two measuring electrodes 87 and 88 are arranged oppositeto one another in a direction normal to the direction of the magneticalternating field which is produced by the two poleshoes 78 and 79 ofthe magnetic core 84. Round the poleshoes 78 and 79 two series connectedcoils 89 and 90 are arranged which are fed with a current in the orderof magnitude of 1 to 4 A, at a tension of 220 V, 50 or 60 Hz. In the airgap between the poleshoes 78, 79 and the tube 86 two series connectedmeasuring coils 91 and 92 are arranged for generating a reference signalwhich is directly proportional to the intensity of the magnetic fUi dl=0is complied with It is known to determine the amplitude and phase of thegenerated field from the current flowing through the coils. For thispurpose, a current transformer is mostly used. This process however,gives rise to considerable error because of eddy currents in the copperas well as in the iron material of the core which leads to the creationof additional fields in the magnetic circuit so that the amplitude andphase of the current is no longer a measure for the generated field.These losses strongly depend on the temperature. Moreover, the eddycurrents in the core iron may give rise to the generation of currentswhich are shifted in phase and which themselves may generate magneticfields which are also shifted in phase.

Hence, according to a preferred feature of the present invention, thephase and the amplitude of the field are measured as close as possibleto the area where the measuring electrodes are situated, i.e. as closeas possible to the measuring tube. For this reason the measuring coils91 and 92 are arranged on both poleshoes 78 and 79 of the magnetic core.Since the applied voltage therein is proportional to (dO/dt) and (MIBdS, the output signal must first be integrated to obtain a measure ofthe field, see formula (2) hereinbefore.

The pre-amplifier according to FIG. 2 comprisesfour operationalamplifiers. The operational amplifiers 10 and 11 have differentialinputs and are connected as feedback amplifiers, and they are eachprovided between their inverting input and their output with a feedbackresistor 20 and 21, of equal resistance value. The non-inverting inputsof the amplifiers 10 and 11 are connected to the measuring electrodes 87and 88. The outputs of the amplifiers l0 and 11 are connected to thedifferential inputs of a third feedback amplifier 12 through theresistors 26 and 27. The reference number 80 stands for the feedbackresistor of the amplifier 12. A resistor 41 is connected between thenoninverting input of the amplifier 12 and the ground. Between theinverting inputs of the amplifiers 10 and 11 two equal resistors 24 and25 are connected in series, while the node of the resistors 24 and 25 isconnected over a capacitor 50 to the input of a fourth operationalamplifier 13 which is connected as a voltage follower. The node of bothidentical resistors 24 and 25 lies at the common mode voltage. Thiscommon mode voltage is fed back by the voltage follower 13, which has alow output impedance, through the identical resistances 22 and 23 to thenon-inverting inputs of current flowing through either the resistors 24,25 orthrough the resistors 20 and 21, so that e e a e., e,,. The commonmode voltage is thus transmitted with an amplification factor 1.

For a differential voltage Ae e e a current i (Ac/2 R flows through theresistors 24 and 25 and consequently also through the resistors 20 and21, so that and since This circuit can reject the common mode by afactor times better than the same amplifier in the usual circuit,wherein A is the amplification factor of the amplifier and 11.

If for example m/R 100, then the rejection is improved by 40 dB. If theoperational amplifier 12 has a rejection of 60 dB in common mode, then arejection of 100 dB is obtained with the pre-amplifier according to FIG.2. In order to determine a path for the bias current, the input of theamplifier is connected to the ground over a low ohmic impedance throughthe resistors 22 and 23 and the voltage follower 13.

Besides a good common mode rejection, it is at least equally importantto have high input impedances in differential mode as well as in commonmode.

The impedance in common mode is especially important because the inputvoltages in common mode may be rather high in industrial arrangements.Input signals in common mode (e e meet an apparently increased impedanceat the input of the amplifier, since In other words, the apparentimpedances 22 and 23 are infinitely increased.

A similar increase in impedance can be obtained for the differentialinput inpedances, and this is the case in the circuit according to FIG.3. In the circuit according to FIG. 2, the said impedance was mainlydetermined by the series connection of 22 and 23. According to theembodiment of FIG. 3,.however, the resistors 22 and 23 are fed from thenode of voltage dividers 30-28 and 31-29, which are fed at one sidethrough capacitors 52 and 53 with the output voltage of thecorresponding amplifiers 10 and 11, and at the other side with theoutput voltage of the voltage follower 13. When the division ratio ofthe resistors 28 and 30 is represented by A, 25 ao)/ 2s then analternating current is flowing through resistor 22.

Consequently, by an appropriate choice of the ratio A/A' the apparentresistance may be made to increase arbitrarily, and may even becomenegative.

When care has been taken that A is somewhat greater than or equal to A,then the stability of the amplifier will never be disturbed since alwaysthe positive input impedance of the amplifier remains. In order tocharge the measuring line of the measurement electrodes as little aspossible, each shunt impedance must be avoided, and consequently alsothe impedance of the necessary shielding.

For this purpose, the two wires which lead from the measuring electrodes87 and 88 to the pre-amplifier are each individually shielded, theshielding for the lead from the measuring electrode 87 to the amplifier10 being fed from the node of the resistors 22 and 28, optionallythrough a resistor 32 (see FIG. 4) and the shielding for the lead fromthe measuring electrode 88 to the amplifier 11 is fed from the node ofthe resistors 23 and 29, optionally through a resistor 33. In this waythe influence of the shunt capacity is reduced as much as the apparentresistance value of the resistor is increased. The resistors 32 and 33assure the highfrequency stability of the circuit of FIG. 4. Thecapacitor 54 serves to prevent occasional oscillation and is selected insuch a way that the bridge formed by the resistors 22 and 28, thedivided capacity of the shielding, and the capacitor 54 is balanced.

A practised embodiment of a pre-amplifier according to FIG. 4 met thefollowing in the temperature range of 30 C:t20 Cz amplification 1600 :10reproducibility better than 0.1

phase error 1 input impedance for common mode voltage 2 input impedancefor differential voltage output alternating voltage lmV /cm3/min i 10output direct voltage 5 100 mV rejection ratio of the common mode betterthan FIG. 5 shows an electronic device for measuring the deviation ofthe rate of flow by means of the measuring device according to FIG. 1,wherein use is made of the pre-amplifier 103 according to FIG. 4.

At 93 the measuring coil for the magnetic alternating field is connectedwith an integrator 95 and also with a potentiometer 96 which allows aphase correction to be made. The signal of the integrator 95 is furtherpassed to a decade-potentiometer 98, by means of which the desired rateof flow (set point value) can be set. The output of potentiometer 98 ispassed to a differential amplifier 100, together with the output of thepre-amplifier 103 according to the invention, which signal may bestandardized and calibrated in part 99.

The operation of the device according to FIG. 5 can be explained asfollows. The output of the pre-amplifier 103 is U S'.B.v,, wherein S isa constant value. By amplifying and integrating the output voltage 93 ofthe measuring coils, one obtains avalue directly proportional to thealternating magnetic field U K.B wherein K is a factor ofproportionality. With the decade-potentiometer 98 this factor ofproportionality can be divided e.g. in such a way that U =S.v .B whereinv,,= the desired value of the velocity (proportional to the rate-offlow) and S.v a.K wherein a is the division factor of the potentiometer.

The signals U and U are subtracted from each other in the differentialamplifier 100 so that U U U This signal is synchronously detected inassembly 101 in order to eliminate error voltages which are shifted over90 with respect to the measuring voltage.

The output signal of the synchronously operating detector is passed to alow pass filter 102. The output signal 1040f the device according toFIG. represents the difference in the rate of flow with respect to theset point and can be used for the operation of a control valve for theliquid.

For measuring and recording, this signal can be used to control theservo-amplifier of a potentiometer recorder wherein the potentiometer ofthis latter instrument is used to replace the set point potentiometer98.

In the same way, use can be made of an analogdigital converter with aforward-backward counter in order to obtain a directly readableindication of the flow by means of numerical display tubes.

The embodiments described above and illustrated in the drawings are, ofcourse, susceptible of numerous modifications without departing from thespirit and scope of the present invention.

We claim:

1. In a magnetic flow meter for measuring the rate of flow of a liquidflowing through a conduit, wherein a magnetic alternating flux isgenerated almost normal to the direction of flow of the liquid throughthe conduit, and a voltage is measured over two measurement electrodeswhich are located on an axis almost normal to the direction of the fluxand to the direction of the How of the liquid, anelectronic circuitwhich comprises a preamplifier with high common mode rejection ratio andhigh input impedance, comprising two identically connected operationalamplifiers having different inputs, each feeding one of the differentialinputs of a third operational amplifier, two identical irnpedancesconnected in series between the inverting inputs of the first twooperational amplifiers, a fourth operational amplifier connected as avoltage follower with its input connected to the node of the said twoimpedances and operational amplifiers, the input terminals of thepreamplifier being formed by the said non-inverting inputs of the twofirst mentioned operational amplifiers and the output of thepre-amplifier being formed by the output of the third operationalamplifier.

2. Magnetic flow meter according to claim 1,

wherein the two electrodes are each connected to the input terminals ofthe pre-arnplifier by a shielded lead, the shielding of each lead beingconnected to the corresponding node of the pairs of series resistors.

3. Magnetic flow meter according to claim 1, wherein means is providedfor comparing the output signal of said pre-amplifier with a signalwhich is proportional to the magnetic flux which actually passes throughthe liquid between the two measurement electrodes.

4. Magnetic flow meter according to claim 3, wherein said proportionalsignal is generated by a measuring coil located close to the two saidelectrodes and connected to an integrator circuit.

Magnetic ow meter accordlng to clalm 3,

wherein said means for comparing the output signal of the pre-amplifierwith a signal which is proportional to the magnetic flux is adifferential amplifier, the output of which is connected to asynchronous demodulator which eliminates voltages which areshifted over90 with respect to the said magnetic flux.

1. In a magnetic flow meter for measuring the rate of flow of a liquidflowing through a conduit, wherein a magnetic alternating flux isgenerated almost normal to the direction of flow of the liquid throughthe conduit, and a voltage is measured over two measurement electrodeswhich are located on an axis almost normal to the direction of the fluxand to the direction of the flow of the liquid, an electronic circuitwhich comprises a preamplifier with high common mode rejection ratio andhigh input impedance, comprising two identically connected operationalamplifiers having different inputs, each feeding one of the differentialinputs of a third operational amplifier, two identical impedancesconnected in series between the inverting inputs of the first twooperational amplifiers, a fourth operational amplifier connected as avoltage follower with its input connected to the node of the said twoimpedances and its output connected through two pairs of seriesresistors to the corresponding non-inverting inputs of the two firstmentioned operational amplifiers, the nodes of said pairs of seriesresistors being connected through further resistors to the output of thecorresponding operational amplifiers, the input terminals of thepre-amplifier being formed by the said non-inverting inputs of the twofirst mentioned operational amplifiers and the output of thepre-amplifier being formed by the output of the third operationalamplifier.
 2. Magnetic flow meter according to claim 1, wherein the twoelectrodes are each connected to the input terminals of thepre-amplifier by a shielded lead, the shielding of each lead beingconnected to the corresponding node of the pairs of series resistors. 3.Magnetic flow meter according to claim 1, wherein means is provided forcomparing the output signal of said pre-amplifier with a signal which isproportional to the magnetic flux which actually passes through theliquid between the two measurement electrodes.
 4. Magnetic flow meteraccording to claim 3, wherein said proportional signal is generated by ameasuring coil located close to the two said eleCtrodes and connected toan integrator circuit.
 5. Magnetic flow meter according to claim 3,wherein said means for comparing the output signal of the pre-amplifierwith a signal which is proportional to the magnetic flux is adifferential amplifier, the output of which is connected to asynchronous demodulator which eliminates voltages which are shifted over90* with respect to the said magnetic flux.